Display device, method of driving a display device, electronic apparatus

ABSTRACT

A display device comprising a driver circuit which modulates the duty cycle of the on-state of a pixel during a frame period. Preferably the driver circuit comprises a comparator and more preferably the comparator is formed of thin film transistors constituting a differential pair and an inverter. Also provided is a method of driving a display device comprising the step of modulating the duty cycle of the on-state of a pixel during a frame period. Beneficially the display device is an organic electroluminescent active matrix display device.

The present invention relates to display devices and in particular toimproving the display quality thereof. The invention also relates to amethod and an electronic apparatus.

One example of a display device to which the present invention relatesis an organic electroluminescent display device. Organicelectroluminescent devices (OELDs) comprise a layer (active layer) oforganic light emitting material, often a light emitting polymer,sandwiched between two electrodes which are used to pass a currentthrough the active material. The device essentially behaves like a diodeand the intensity of light emission is a function of the forward biascurrent which is applied. The devices are good candidates for thefabrication of display panels.

A basic requirement for a display panel is an ability to display goodquality graphical images. This is dependent upon the ability of theindividual pixels to generate a range of brightness intensity. The imagequality improves as the number of gray scales increases. Theconventionally used standard is 3×8 bit colour, equivalent to 256 grayscales per colour. This standard is used in many current dayapplications.

Various methods of generating gray scales with an analog driving circuithave been proposed for OELD displays. The conventional technique is todrive the OELD with a voltage dependent current and this has allowed theimplementation of active matrix OELD displays. A typical arrangement isillustrated in FIG. 1 hereof.

As shown in FIG. 1, when transistor T₁ is selected (by voltage V_(sel))it turns on and the data voltage (V_(dat)) is transferred to the gate oftransistor T₂. Assuming T₂ is biased in the saturation region, the datavoltage V_(dat) is converted into current, which drives the OELD to therequired brightness intensity.

The variation of threshold voltages of the transistors is, however, avery significant problem in the practical implementation of the abovedescribed display panels. Another significant problem is the high powerconsumption of these circuits.

An alternative method of providing gray scaling is to use an areadithering technique in which each pixel is divided in to a number ofsub-pixels, preferably with binary weighted areas. Each sub-pixel isdriven either fully on or fully off. Thus a digital driver can be usedand power consumption reduced. However, this technique has thedisadvantage that the panel size is increased (because each pixel isreplaced by a number of sub-pixels and, in the limit, each sub-pixel isthe same size as a conventional pixel) and also there is a largeincrease in the number of signal lines required (because of the need toaddress each sub-pixel).

Against this background, it is an object of the present invention toprovide a display device with good gray scale capabilities whichmitigates the above mentioned disadvantages.

According to the present invention there is provided a display devicecomprising a driver circuit which modulates the duty cycle of theon-state of a pixel during a frame period.

Thus, the present invention provides pulse width modulation of theon-period of a pixel and the integrating function of the human eyeperceives this as modulation of the intensity of the emitted light.Modulation of the on-period is in stark contrast to the conventionalcontrol of brightness, ie control of the instantaneous amplitude of thecurrent supplied.

Embodiments of the present invention will now be described in moredetail by way of further example only and with reference to theaccompanying drawings, in which:

FIG. 1 is a circuit diagram of a conventional pixel level driver in anOELD display panel;

FIG. 2 is a circuit diagram of a pixel level driver in an OELD displaypanel, according to one embodiment of the present invention;

FIG. 3 illustrates a detailed circuit diagram and operating waveformsfor an implementation of the comparator shown in the circuit of FIG. 2;

FIG. 4 illustrates driving waveforms in the circuit of FIG. 2;

FIG. 5 is a circuit diagram illustrating the use of an integratedwaveform generator;

FIG. 6 illustrates a generalised synchronous driving scheme;

FIG. 7 illustrates a generalised asynchronous driving scheme;

FIGS. 8A and 8B show the significance of using higher frequencies in theasynchronous driving scheme;

FIGS. 9A and 9B illustrate the incorporation of gamma correction in tothe driving voltage;

FIG. 10 is a detailed circuit diagram of a sawtooth wave generator;

FIG. 11 shows input waveforms for the circuit of FIG. 10;

FIGS. 12A and 12B show gray scales obtained in a specific example;

FIG. 13 is a schematic view of a mobile personal computer incorporatinga display device having a pixel driver according to the presentinvention,

FIG. 14 is a schematic view of a mobile telephone incorporating adisplay device having a pixel driver according to the present invention,and

FIG. 15 is a schematic view of a digital camera incorporating a displaydevice having a pixel driver according to the present invention,

A description will first be given of the pixel level configurationaccording to one embodiment of the present invention. Thus, FIG. 2 is acircuit diagram of an individual pixel 10 within an active matrix OELDdisplay panel. The circuit is implemented using polysilicon TFTcomponents and comprises an MOS-input comparator 12 and two pass-gates,SW₁ and SW₂. The use of pass-gates avoids so-called “feed-through”, i.e.coupling with other circuit voltages. The inverting input (+) of thecomparator 12 is connected to a waveform source V_(saw). Thenon-inverting input (−) is connected to a storage capacitor C₁ and apass-gate SW₁. The pass-gate SW₁ is controlled by a waveform V_(sel).The output of the comparator is connected to a pass-gate SW₂. Pass-gateSW₂ controls the current flowing in to the organic light emittingelement 14. By applying a time varying signal to V_(saw), the lightemitting element 14 is switched on for a period depending on the valueof the data voltage V_(dat) which is applied to the other side ofpass-gate SW₁ compared to the capacitor C₁ and the comparator 12.

In a line-at-a-time driving scheme, V_(sel) sets the state of thepass-gate SW₁ of the pixel elements on the same row. When pass-gate SW₁is closed, the data voltage V_(dat) is transferred to the invertinginput of the comparator 12 and to the capacitor C₁. Then, when pass-gateSW₁ is opened the data voltage is memorised by capacitor C₁. Thewaveform V_(saw) is then initiated. When the voltage, V₊, at theinverting input of the comparator 12 is less than the voltage, V−, atthe non-inverting input thereof, the comparator outputs a LO signalwhich puts the light emitting element 14 in to the on-state. When thevoltage, V₊, at the inverting input of the comparator 12 is greater thanthe voltage, V−, at the non-inverting input thereof, the comparatoroutputs a HI signal which puts the light emitting element 14 in to theoff-state. As a result the data voltage stored by the capacitor C₁modulates the duration for which the light emitting element 14 remainsin the on-state during a frame period.

The frame period might typically be 20 mS and with the response time ofthe light emitting element 14 being of the order of nano-seconds, thespeed of the polysilicon TFTs and any stray capacitance become thelimiting factors in operation of the driving scheme. That is,exceptionally effective switching can be obtained.

In the circuit illustrated in FIG. 2, a common operating voltageV_(OELD) is used for all OELD pixels of the same type. The voltageV_(OELD) is set externally and is independent of the supply voltageV_(DD) of the driving circuit. This significantly increases theflexibility of controlling the bias conditions for the OELDs.

A description will now be given of the detailed considerations whichapply to the practical implementation of the comparator 12 used in thecircuit of FIG. 2.

Since a separate comparator is provided for each pixel, the circuit areaand power consumption of the comparator should be kept as low aspossible. Further, in order to achieve a high number of gray scales, thecomparator must be able to distinguish a small difference in inputvoltages. For example, if it is desired to implement 256 gray scaleswith a voltage swing of 0V to 5V then clearly something of the order ofΔV=19.5 mV is appropriate. Thus switching must be very fast but, fromthe previous discussion, it is well within the ability of the describedcircuit.

A detailed circuit diagram of one implementation of the comparator 12 ofFIG. 2 is illustrated in FIG. 3. The circuit of FIG. 3 comprises twostages: a CMOS differential amplifier 16, and a CMOS inverter 18. TheCMOS inverter 18 turns the pass-gate SW₂ fully on or fully off veryquickly. For level shifting purposes the power supply of the inverterstage 18 can be different from that of the differential stage 16.

The differential stage 16 comprises the drain-source series connectioncircuit of transistors 20, 21 and 23 connected between the V_(DD) railand ground, together with the similarly connected circuit of transistors20, 22 and 24, wherein transistors 22 and 24 are connected in parallelwith transistors 21 and 23. The respective gates of transistors 21 and22 provide the two input terminals (+), (−) of the comparator 12,whereas the gate of transistor 20 receives a bias voltage V_(bias). Theoutput stage 18 comprises two transistors, 25 and 26, which aresource-drain series connected between the V_(DD) rail and ground. Theoutput V_(out) of the comparator is taken from the connection betweenthe transistors 25 and 26 and the gates thereof receive there input fromthe junction between transistors 21 and 23.

The circuit illustrated in FIG. 3 uses seven TFTs. Using a respectiveTFT for SW₁ and SW₂ brings the total per pixel to nine.

A description will now be given of various aspects of implementing adisplay panel incorporating the above described embodiment of pixellevel circuitry.

FIG. 4 illustrates waveforms which can be used with the circuit of FIG.2. FIG. 4 comprise two diagrams, (a) and (b), in which the waveformsV_(scan), V_(saw) and V_(out) are shown. V_(out) is the driving pulseapplied to the OELD. FIGS. 4( a) and (b) differ in the shape of thewaveform used for V_(saw). In FIG. 4( a) the waveform of V_(saw) is asawtooth whereas in FIG. 4( b) the waveform of V_(saw) is triangular.Using the sawtooth waveform of FIG. 4( a) the output pulse always startsat the beginning of each frame. Thus the sawtooth waveform of FIG. 4( a)provides a linear gray scale, as it provides a reference time point forthe eye to start integrating for each frame. For the triangular waveformof FIG. 4( b) the centre of the output pulse always occurs at mid-cycle.

Basically all pixels in the same row of the matrix share the samedriving waveform, denoted by V_(saw/m) where m indicates that it is them^(th)-row of the matrix which is being considered. When rows areaddressed sequentially, the driving waveforms for the next row, denotedby V_(saw/m+1), should incorporate a delay or phase shift ofT_(frame)/M, where T_(frame) is the frame period and M is the totalnumber of rows in the matrix. Thus if the display is driven externally atotal of M interconnections are required. This can be a problem for highresolution displays. Thus, according to one embodiment of the presentinvention there is provided an integrated waveform generator, by whichthe number of interconnections required can be reduced.

FIG. 5 is a circuit diagram illustrating the use of an integratedwaveform generator. The waveform generator 30 receives separate masterand reference voltage inputs, V_(master) and V_(ref). The waveformgenerator 30 also receives an input from V_(scan/m). The generatoroutput V_(saw/m) is applied to all of the pixels 10 in a particular rowof the matrix.

Ideally, however, the function of the generators is to provide the samewaveform with a unique phase shift for each row of pixel elements. Theprecise timing and data voltage relationship becomes a major challengewhen the spatial variation of TFT characteristics over a display panelis taken into account. However, this problem can be solved by providingthe master clock V_(master) and the reference voltage source V_(ref) toensure that outputs from all waveform generators are the same butdifferent in phase shift.

The waveform generator should be synchronised to V_(scan/m), and thusthe signal V_(scan/m) can be used as a trigger.

From the foregoing description, a generalised synchronous driving schemeis illustrated in FIG. 6. Two rows and six columns of pixels areillustrated. As denoted by R, G, B indicating red, green and blue; thelight emitting element in each pixel may be designed to emit light ofdifferent colours thus implementing a full colour display. The pixelsare driven by a data driver 32 and a row driver 34. A separate waveformgenerator, WG, is provided for each row and the signals applied areindicated in FIG. 6. Each waveform generator is synchronised to the scanline signal and the minimum operating frequency is equal to the framerate.

The display can also be driven asynchronously. An asynchronous drivingscheme is shown in FIG. 7. The difference between this arrangement andthat illustrated in FIG. 6 is that a single waveform generator is usedfor the whole display rather than using one per row. With thisarrangement the waveform generator can be integrated on the displaypanel or can easily be provided externally of the panel. The waveform isindependent of the scan line signal and higher operating frequencies canthus be used, thereby obtaining better image quality. The significanceof using higher frequencies can be appreciated from FIGS. 8A and 8B,that is the improved gray scale accuracy of FIG. 8B (high frequencyV_(DRV)) compared with FIG. 8A (low frequency V_(DRV)) is readilyapparent. This phenomenon is important for moving images but caneffectively be ignored for still images.

It is also possible to incorporate gamma compensation into the drivingwaveform. This is illustrated in FIGS. 9A and 9B, which show gammacorrection incorporated in to the driving voltage V_(DRV).

FIG. 10 is a detailed circuit diagram of a sawtooth waveform generatorsuch as may be employed in the above described embodiments of thepresent invention. The circuit receives an input signal V_(gray) whichis applied to one terminal of a capacitor C₂₀. The other terminal ofcapacitor C₂₀ is connected to one side of each of switches SW₁₀ andSW₂₀. These switches SW₁₀ and SW₂₀ are controlled by signals φ₁ and φ₂,respectively. The other side of switch SW₂₀ is connected to ground via acapacitor C₁₀ and also via a switch SW₃₀ which is controlled by signalV_(scan). Switches SW₂₀, SW₃₀ and capacitor C₁₀ are connected to theinput of a unity gain buffer 36. Switch SW₁₀ controls a feedback loopfrom the output of the buffer 36. The output of the buffer 36 is appliedto a low-pass filter L.P. consisting of a resistor and a capacitor. Theout put of the filter L.P. provides the generator output V_(saw).

As noted above, the circuit has four inputs (V_(gray), φ₁, φ₂ andV_(scan)) and one output (V_(saw)). The input waveforms are shown inFIG. 11.

Waveform V_(gray) operates between 0V and a maximum level, say h.Waveforms φ₁ and φ₂ are non-overlapping clock pulses and V_(scan) is thesame signal as in the scan line. When V_(scan) goes HI, data istransferred to the pixel storage capacitor as described above. At thesame time, V_(scan) signals SW₃₀ to close so that the input of the unitygain buffer is at 0V and C₁₀ is discharged. Effectively, this acts as areset and zeros the output. When V_(scan) goes LO, SW₃₀ is opened.Waveform V_(gray)=0V when SW₂₀ is closed and SW₁₀ is opened. Thetransition of V_(gray) from 0V to h raises the input voltage at theunity gain buffer. If C₁₀=C₂₀, this increment equals h/2. WhenV_(gray)=h, SW₂₀ is opened and SW₁₀ is closed. The unity gain buffer 32input voltage is stored by C₁₀. This voltage is reflected by the outputof the unity gain buffer and is connected to C₂₀ while V_(gray) returnsto 0V. Next SW₁₀ is opened and then SW₂₀ is closed, and then V_(gray)will transit from 0V to h. This will increase further the voltage at theinput of the unity gain buffer 32. If C₁₀=C₂₀, this increment equals h/2and the resulting voltage becomes h. This continues and the output ofthe unity gain buffer 36 takes on a step shape. If the output is passedthrough the low pass filter L.P. the output signal becomes a smoothramp.

It may be appreciated that the described arrangements according to thepresent invention can utilise existing analog video signals as inputsignals.

EXAMPLE

An example was implemented using the circuits described above, withpolysilicon TFTs. Using a data voltage range of 0V to 5V, 256 grayscales were implemented.

After the data transfer, which typically occurs in the first 20 μs, theframe period was divided into 256 sections. For a frame rate of 50cycles/s, the time difference for each additional gray scale is given byΔt=1/50÷256=78.125 μs, and the corresponding data voltage difference isgiven by ΔV=5÷256=19.53 mV. It is noted that for gray scale=0 the OELDmust not be turned on at all.

FIGS. 12A and 12B show the first five (GS=1 to 5) and last five (GS=252to 256) gray scales, respectively. The area under the pulses arecalculated and plotted against the gray scale. As shown in FIGS. 12A and12B, there is good linearity of pixel brightness within the grayscaling. However, a difference in slope is noted. This is believed to bedue to the round corner in the pulse trailing edges, caused by thecircuit's stray capacitance. This results in a smaller change inbrightness for the lower gray scale values. This is not a seriousproblem and can be corrected by adjusting the input signal.

The current required by the driver is small compared to the currentflowing in to the electroluminescent element.

Generally, the image quality which can be achieved with the presentinvention has been found to be superior to conventional Liquid CrystalDisplays and at least equal to that of conventional CRT displays. Inaddition, the low power consumption required by the display device ofthe present invention makes it ideal for mobile and portable apparatus.

Modifications

As will already be appreciated, although much of the detail given abovein relation to specific embodiments has been in terms of organicelectroluminescent display devices; the present invention is alsoapplicable to other types of display devices. Further, althought theabove described embodiments have mentioned specific implementation usingTFT technology, usually in polysilicon,; the present invention is notlimited to the use of TFT technology. The invention is applicable notonly to thin film transistor technology but also to silicon basedtransistors. Silicon based transistors can be arranged on a displaysubstrate using several different methods. For example, silicon basedtransistors can be arranged in a liquid.

The present invention is advantageous for use in small, mobileelectronic products such as mobile phones, computers, CD players, DVDplayers and the like—although it is not limited thereto.

Several electronic apparatuses using a display device according to thepresent invention will now be described.

<1: Mobile Computer>

An example in which the display device according to one of the aboveembodiments is applied to a mobile personal computer will now bedescribed.

FIG. 13 is an isometric view illustrating the configuration of thispersonal computer. In the drawing, the personal computer 1100 isprovided with a body 1104 including a keyboard 1102 and a display unit1106. The display unit 1106 is implemented using a display panelfabricated according to the present invention, as described above.

<2: Portable Phone>

Next, an example in which the display device is applied to a displaysection of a portable phone will be described. FIG. 14 is an isometricview illustrating the configuration of the portable phone. In thedrawing, the portable phone 1200 is provided with a plurality ofoperation keys 1202, an earpiece 1204, a mouthpiece 1206, and a displaypanel 100. This display panel 100 is implemented using a display panelfabricated according to the present invention, as described above.

<3: Digital Still Camera>

Next, a digital still camera using an OEL display device as a finderwill be described. FIG. 15 is an isometric view illustrating theconfiguration of the digital still camera and the connection to externaldevices in brief.

Typical cameras sensitize films based on optical images from objects,whereas the digital still camera 1300 generates imaging signals from theoptical image of an object by photoelectric conversion using, forexample, a charge coupled device (CCD). The digital still camera 1300 isprovided with an OEL element 100 at the back face of a case 1302 toperform display based on the imaging signals from the CCD. Thus, thedisplay panel 100 functions as a finder for displaying the object. Aphoto acceptance unit 1304 including optical lenses and the CCD isprovided at the front side (behind in the drawing) of the case 1302.

When a cameraman determines the object image displayed in the OELelement panel 100 and releases the shutter, the image signals from theCCD are transmitted and stored to memories in a circuit board 1308. Inthe digital still camera 1300, video signal output terminals 1312 andinput/output terminals 1314 for data communication are provided on aside of the case 1302. As shown in the drawing, a television monitor1430 and a personal computer 1440 are connected to the video signalterminals 1312 and the input/output terminals 1314, respectively, ifnecessary. The imaging signals stored in the memories of the circuitboard 1308 are output to the television monitor 1430 and the personalcomputer 1440, by a given operation.

Examples of electronic apparatuses, other than the personal computershown in FIG. 13, the portable phone shown in FIG. 14, and the digitalstill camera shown in FIG. 15, include television sets, view-finder-typeand monitoring-type video tape recorders, car navigation systems,pagers, electronic notebooks, portable calculators, word processors,workstations, TV telephones, point-of-sales system (POS) terminals, anddevices provided with touch panels. Of course, the above describedembodiments of the present invention can be applied to the displaysections of these electronic apparatuses.

1. A display device, comprising: a plurality of scanning lines; aplurality of data lines; a plurality of time varying signal lines; aplurality of pixels, each of which includes: a first pass-gate that iscontrolled by one scanning line of the plurality of scanning lines; acapacitor; a second pass-gate that is connected to a common operatingvoltage; and a comparator that has a first input, a second input and anoutput, the first input and the second input being connected to thecapacitor and one time varying signal line of the plurality of timevarying signal lines, respectively, the capacitor storing a data signalsupplied through the first pass-gate and one data line of the pluralityof data lines, the comparator performing comparison of the data signalstored by the capacitor with a time varying signal supplied from the onetime varying signal line and outputting an output signal on the basis ofthe comparison, the time varying signal incorporating a gammacorrection; the output signal being supplied to a gate of the secondpass-gate, and the second pass-gate controlling a current supply fromthe common operating voltage according to the output signal.
 2. Thedisplay device according to claim 1, the comparator being formed of thinfilm transistors.
 3. The display device according to claim 2, the thinfilm transistors being formed of polysilicon.
 4. The display deviceaccording to claim 1, the comparator including a differential amplifierand an inverter.
 5. The display device according to claim 1, each of theplurality of pixels further including a light emitting element, and thesecond pass-gate controlling the current supply from the commonoperating voltage to the light emitting element.
 6. The display deviceaccording to claim 5, the light emitting element being connected to thesecond pass-gate.
 7. The display device according to claim 1, the secondpass-gate being connected to a common operating voltage line whosevoltage is set at the common operating voltage.
 8. A driving method fora display device including a plurality of pixels, the driving methodcomprising: supplying a data signal to a capacitor included in one pixelof the plurality of pixels through one data line of a plurality of datalines and a first pass-gate that is included in the pixel; comparing thedata signal stored by the capacitor to a time varying signal thatincorporates a gamma correction and that is supplied through one timevarying signal line of a plurality of time varying signal linesintersecting the plurality of data lines; outputting an output signalthat is obtained by the comparing of the data signal to the time varyingsignal to a second pass-gate included in the one pixel; and supplying acurrent to a light emitting element included in the one pixel throughthe second pass-gate from a common operating voltage according to theoutput signal.
 9. The display device according to claim 1, a drivingcircuit supply voltage being supplied to the comparator.
 10. The displaydevice according to claim 1, further comprising: a plurality of commonoperating voltage lines; and a plurality of driving circuit supplyvoltage lines, the second pass-gate being connected to one commonoperating voltage line of the plurality of common operating voltagelines whose voltages are set at the common operating voltage, and adriving circuit supply voltage being supplied to the comparator throughone driving circuit supply voltage line of the plurality of drivingcircuit supply voltage lines.
 11. A display device, comprising: aplurality of scanning lines; a plurality of data lines; a plurality oftime varying signal lines; a plurality of pixels each of which includes:a first transistor that is controlled by one scanning line of theplurality of scanning lines; a capacitor; a second transistor that isconnected to a common operating voltage; and a comparator that has afirst input, a second input and an output, the first input and thesecond output being connected to the capacitor and one time varyingsignal line of the plurality of time varying signal lines, respectively,the capacitor storing a data signal supplied through the firsttransistor and one data line of the plurality of data lines, thecomparator performing comparison of the data signal stored by thecapacitor with a time varying signal supplied from the one time varyingsignal line and outputting an output signal on the basis of thecomparison, the time varying signal incorporating a gamma correction;the output signal being supplied to a gate of the second transistor, andthe second transistor controlling a current supply from the commonoperating voltage according to the output signal.
 12. The display deviceaccording to claim 11, the plurality of time varying signal linesintersecting the plurality of data lines.
 13. The display deviceaccording to claim 1, the plurality of time varying signal linesintersecting the plurality of data lines.
 14. The display deviceaccording to claim 11, further comprising: a plurality of commonoperating voltage lines; and a plurality of driving circuit supplyvoltage lines, the second pass-gate being connected to one commonoperating voltage line of the plurality of common operating voltagelines whose voltages are set at the common operating voltage, and adriving circuit supply voltage being supplied to the comparator throughone driving circuit supply voltage line of the plurality of drivingcircuit supply voltage lines.
 15. The display device according to claim11, further comprising: a waveform generator that provides a pluralityof time varying signals through the plurality of time varying signallines each of which is generated by performing a phase shift.
 16. Thedisplay device according to claim 1, further comprising: a waveformgenerator that provides a plurality of time varying signals through theplurality of time varying signal lines each of which is generated byperforming a phase shift.
 17. A display device comprising: a pluralityof scanning lines; a plurality of data lines; a plurality of pixels eachof which includes a light emitting element and a capacitor storing adata signal supplied through one data line of the plurality of datalines; a comparison of the data signal stored by the capacitor with atime varying signal that incorporates a gamma correction being carriedout; and a length of the period in which the light emitting elementemits a light depending on the result of the comparison.
 18. The displaydevice according to claim 17, each of the plurality of pixels furtherincluding a comparator that performs the comparison.
 19. The displaydevice according to claim 17, further comprising a plurality of timevarying signal lines, the time varying signal being supplied through onetime varying signal line of the plurality of time varying signal lines.20. The display device according to claim 18, each of the plurality ofpixels further including a pass-gate that controls a current supply froma common operating voltage according to the result of the comparison,and the comparator outputting an output signal that is supplied to agate of the pass-gate.